Using the undefined variable `num`, the code `num += 5` inside of a function gets a different error message

[hyperkai]

Bug report

Bug description:

Using the undefined variable num, the code print(num) outside and inside of a function and num += 5 outside of a function gets the same error messages as shown below:

# num = 10

print(num)
# NameError: name 'num' is not defined

num += 5
# NameError: name 'num' is not defined

def func():
    # num = 10
    print(num)

func()
# NameError: name 'num' is not defined

But using the undefined variable num, the code num += 5 inside of a function gets the different error message as shown below:

def func():
    # num = 10
    num += 5

func()
# UnboundLocalError: cannot access local variable 'num' where it is not associated with a value

So, the error message just above should also be the same as shown below:

NameError: name 'num' is not defined

CPython versions tested on:

3.12

Operating systems tested on:

Windows

[JelleZijlstra]

This is not a bug, it's a consequence of how function scopes work. You can reference the Python documentation for details.

More generally, you're opening a lot of issues that often aren't valid. Please be more selective before opening an issue.

[hyperkai]

Does the error message below mean that num is defined but doesn't have any value in the function? If so, I understand and agree. But if the error message below means that num is not defined at all, it should be NameError: name 'num' is not defined which is simple and consistent.

UnboundLocalError: cannot access local variable 'num' where it is not associated with a value

[JelleZijlstra]

num += num is an assignment statement that assigns to num. That means num is a local variable in the function.

[hyperkai]

You don't answer my question.

OK, so the error message should be like below which is simple and consistent:

NameError: the local variable 'num' is not defined

[terryjreedy]

@hyperkai Please stop using the issue tracker for tutorial help. Use stackoverflow.com or discuss.python.org, help channel.

[skirpichev]

You don't answer my question.

I will try.

error message should be like below which is simple and consistent

But it just plain wrong. Consider this:

>>> num = 1
>>> def f():
...     num += 1
...     
>>> f()
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    f()
    ~^^
  File "<python-input-1>", line 2, in f
    num += 1
    ^^^
UnboundLocalError: cannot access local variable 'num' where it is not associated with a value

You are trying to sell us "NameError: name 'num' is not defined" in issue description. Now it seems you already understand, that it's wrong, as shows this example.

Then your version was "NameError: the local variable 'num' is not defined". But this time it will be a wrong error type and a message. The 'num' is defined, just not in the function block. UnboundLocalError is a subclass of NameError, it gives you more narrower type of error conditions.

Documentation precisely describes this behavior:

1. When a name is not found at all, a NameError exception is raised.
2. If the current scope is a function scope, and the name refers to a local variable that has not yet been bound to a value at the point where the name is used, an UnboundLocalError exception is raised.